The gradual acceptance of hydraulic brake systems in the rapid transit segment of the railway industry suggests the possibility of using hydraulics as a means of transmitting brake forces to the brake shoes in freight-type service. Such an approach would be advantageous in eliminating the need for the cumbersome mechanical brake rigging presently employed on freight cars to transmit the brake forces to the brake shoes. For example, in truck-mounted brake systems, two brake beams are employed per truck, and each beam carries a pneumatic brake cylinder which is interconnected to the adjacent beam of the same truck by a push rod. In order to achieve the desired brake forces, these pneumatic cylinders are relatively large and heavy. Accordingly, the beams that carry these brake cylinders must be specially designed with sufficient strength to support the brake cylinder. This precludes the use of standard, low-cost brake beams typically used in truck-mounted brake systems. By employing relatively small hydraulic brake actuators instead of large pneumatic cylinders to actuate the brake beams, a considerable weight-reduction can be realized, making it possible to utilize light-weight brake beams. Such an arrangement is shown in FIG. 1 of the accompanying drawings, wherein a single hydraulic brake actuator 1 is mounted on one of a pair of light-weight brake beams 2a and 2b having brake shoes 3 carried thereon for engagement with the truck wheel treads. The brake actuator 1 may be mounted, for example, by connecting the actuator body 6 to one arm of a bellcrank lever 4a that is pivotally-attached to brake beam 2b, while the actuator push rod 7 is connected to one arm of another bellcrank lever 4b that is also pivotally-attached to brake beam 2b. The other arm of the respective bellcrank levers 4a and 4b is then connected to the other brake beam 2a by push rods 5a and 5b.
Moreover, a hydraulic-type brake actuator lends itself to a vastly different parking brake concept, which would permit elimination of the conventional, often difficult to operate, handwheel-operated parking brake.
The hydraulic brake actuator contemplated by the present invention can be kept small in size, by reason of the fact that hydropneumatic converters are known having force-multiplication for producing the high hydraulic pressure required.
The converter device 8 includes a relatively large pneumatic piston 9, and a relatively small hydraulic piston 10 that is connected to the pneumatic piston. The hydraulic piston 10 operates in a hydraulic cylinder 11 to displace hydraulic fluid therefrom to the hydraulic system via a port 12, as the pneumatic/hydraulic piston is actuated. The converter device 8 further includes a slack adjuster piston 14 that operates between end stops 15, 16 in a hydraulic reference chamber 17, as the piston assembly is actuated through its nominal stroke, thereby displacing a predetermined amount of hydraulic fluid sufficient to take up the brake shoe clearance. Should overtravel occur due to brake shoe wear, a first one-way check valve 18 is unseated by engagement of the slack adjuster piston 14 with a pin 12a at the one end stop 16, thereby opening a passageway through the slack adjuster piston 14 via which any additional hydraulic fluid required to maintain brake shoe engagement is supplied from the hydraulic cylinder 11.
During a subsequent brake release, the slack adjuster piston 14 is retracted into engagement with the other end stop 15 to establish the desired brake shoe clearance, following which, continued movement of the piston assembly to release position, corresponding to overtravel during the preceding brake application, draws makeup hydraulic fluid into the hydraulic cylinder 11 from the hydraulic reservoir 19 via a second one-way check valve 20.
In the event brake shoe engagement occurs prematurely, i. e., prior to completion of the first stage of operation, a high pressure is developed in the reference chamber 17, due to brake shoe engagement occurring before complete displacement of hydraulic fluid has taken place. Accordingly, a one-way check valve 21a, between the reference chamber 17 and hydraulic reservoir 19, is opened by the hydraulic pressure buildup, to thereby allow the excess hydraulic fluid in the reference chamber 17 to be displaced into the reservoir 19, until the slack adjuster piston 14 engages end stop 16. During a subsequent brake release, the brake shoes will be retracted as the slack adjuster piston 14 is returned to its original position--against its other end stop 15--thereby establishing the desired brake shoe clearance.
A single hydropneumatic converter device, such as that just described, can therefore be utilized to deliver relatively high hydraulic pressures to light-weight hydraulic brake actuators 1 mounted either on or between standard brake beams 2a and 2b of a railway car truck, as shown in FIG. 1.